A system based on the MPEG-4 (Moving Picture Experts Group Phase-4) which is currently in progress toward standardization in ISO/IEC JTC11/SC29/WG11 differs from a system based on ITU-T Recommendation H.263 in header information (an information signal for decoding) to be added to a coded bit stream which constitutes a transmitted signal.
FIG. 1(a) is a diagram showing a structure of an H.263 coded bit stream 201 based on the H.263 standard, into which header information 211 is multiplexed along with macroblock data 225 consisting of image coded data encoded according to the H.263 coding scheme. FIG. 1(b) is a diagram showing a structure of an MPEG-4 coded bit stream 202, into which header information 212 is multiplexed along with macroblock data 239 consisting of image coded data encoded according to the MPEG-4 coding scheme. As shown in these figures, they have different coded bit stream structures. In particular, the H.263 does not include header information about VO (Video Object), VOL (Video Object Layer), VOP (Video Object Plane) and the like, which are required for MPEG-4 decoding. Accordingly, to carry out the image communications based on the two schemes, separate image decoding apparatuses and image coding apparatuses are needed.
Here, it is not always necessary for a GOB start code 223 and GOB header information 224 in the H.263 coded bit stream 201, and resynchronization instruction code 237 and resynchronization information 238 in the MPEG-4 coded bit stream 202 to be inserted, but inserted as needed.
With such structures, the conventional coded bit streams present a problem in that an MPEG-4 compatible image decoding apparatus, for example, cannot decode the H.263 coded bit stream 201 generated according to the H.263 standard.
Furthermore, to decode the coded bit streams according to the MPEG-4 and H.263 standard, an image decoding apparatus must comprise two decoders based on the two schemes, which presents a problem of complicating the apparatus.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide an image decoding apparatus capable of decoding the H.263 coded bit stream 201, an image coding apparatus for generating a coded bit stream decodable by the image decoding apparatus, and an image communications system and a coded bit stream converting apparatus for converting the H.263 coded bit stream to the MPEG-4 coded bit stream to carry out communication, all of which apparatuses have a simple structure.